Commit 36c682f6 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet
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DMA-attributes.txt: standardize document format



Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

This file is almost on the standard format. All it needs
to be parsed by Sphinx is to fix its main title and add
a few blank lines.

Yet, it is worth to replace NOTE: by the corresponding
ReST markup.

While here, remove the extra "---" from a subtitle.
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 266921bd
DMA attributes ==============
============== DMA attributes
==============
This document describes the semantics of the DMA attributes that are This document describes the semantics of the DMA attributes that are
defined in linux/dma-mapping.h. defined in linux/dma-mapping.h.
...@@ -108,6 +109,7 @@ This is a hint to the DMA-mapping subsystem that it's probably not worth ...@@ -108,6 +109,7 @@ This is a hint to the DMA-mapping subsystem that it's probably not worth
the time to try to allocate memory to in a way that gives better TLB the time to try to allocate memory to in a way that gives better TLB
efficiency (AKA it's not worth trying to build the mapping out of larger efficiency (AKA it's not worth trying to build the mapping out of larger
pages). You might want to specify this if: pages). You might want to specify this if:
- You know that the accesses to this memory won't thrash the TLB. - You know that the accesses to this memory won't thrash the TLB.
You might know that the accesses are likely to be sequential or You might know that the accesses are likely to be sequential or
that they aren't sequential but it's unlikely you'll ping-pong that they aren't sequential but it's unlikely you'll ping-pong
...@@ -121,11 +123,12 @@ pages). You might want to specify this if: ...@@ -121,11 +123,12 @@ pages). You might want to specify this if:
the mapping to have a short lifetime then it may be worth it to the mapping to have a short lifetime then it may be worth it to
optimize allocation (avoid coming up with large pages) instead of optimize allocation (avoid coming up with large pages) instead of
getting the slight performance win of larger pages. getting the slight performance win of larger pages.
Setting this hint doesn't guarantee that you won't get huge pages, but it Setting this hint doesn't guarantee that you won't get huge pages, but it
means that we won't try quite as hard to get them. means that we won't try quite as hard to get them.
NOTE: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM, .. note:: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
though ARM64 patches will likely be posted soon. though ARM64 patches will likely be posted soon.
DMA_ATTR_NO_WARN DMA_ATTR_NO_WARN
---------------- ----------------
...@@ -142,10 +145,10 @@ problem at all, depending on the implementation of the retry mechanism. ...@@ -142,10 +145,10 @@ problem at all, depending on the implementation of the retry mechanism.
So, this provides a way for drivers to avoid those error messages on calls So, this provides a way for drivers to avoid those error messages on calls
where allocation failures are not a problem, and shouldn't bother the logs. where allocation failures are not a problem, and shouldn't bother the logs.
NOTE: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC. .. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
DMA_ATTR_PRIVILEGED DMA_ATTR_PRIVILEGED
------------------------------ -------------------
Some advanced peripherals such as remote processors and GPUs perform Some advanced peripherals such as remote processors and GPUs perform
accesses to DMA buffers in both privileged "supervisor" and unprivileged accesses to DMA buffers in both privileged "supervisor" and unprivileged
......
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